Minivan owners reported real-world fuel economy that was equal or slightly lower than the EPA ratings.

Driver behaviors and environmental conditions, rather than vehicle shortcomings, are likely responsible for most fuel economy variances, the study found.

"If you drive aggressively, with heavy acceleration, hard braking and driving at higher speeds, your fuel economy is going to suffer," said John Nielsen, AAA's managing director of automotive engineering and repair. "Driving just five miles-per-hour above 50 is like paying an additional 19 cents per gallon for gasoline."

The findings came after a comprehensive review by AAA engineers of 37,000 records submitted to the EPA, representing more than 8,400 vehicle make, model, and year combinations.

In conducting this analysis, AAA engineers identified a list of vehicles that were frequently reported as failing to achieve the EPA's mileage rating. The majority of these vehicles, including the scrutinized Hyundai and Kia models, have since been retested and, in some cases, mileage ratings were revised. AAA selected three additional vehicles — a 2014 full-size pickup truck, a 2014 large sedan and a 2012 medium sedan — for further testing.